Device for exciting surface waves



Oct. 13, 1970 H. JOUFFROY ET AL 3,534,300

DEVICE FOR EXCITING SURFACE WAVES Filed June 9, 1966 2 Sheets-Sheet 1 Oct. 13, 1970 JQUFFROY ETAL 3,534,300

DEVICE FOR EXCITING SURFACE WAVES 2 Sheets-Sheet 2 Filed June 9. 1966 3,534,300 DEVICE FOR EXCITING SURFACE WAVES Henri Jouifroy and Jean Desbois, Paris, France, assignors to Thomson C.S.F., Paris, France, a corporation of France Filed June 9, 1966, Ser. No. 556,419 Claims priority, application France, June 16, 1965,

21,00 Int. Cl. H03h 7/30 U.S. Cl. 333-30 9 Claims ABSTRACT OF THE DISCLOSURE The present invention relates to acoustic lines used in telecommunication systems with a view to treating and more particularly to compressing electric signals. -It relates more particularly to coupling devices used for exciting in or recovering from an electro-acoustic transducer, a surface wave having a predetermined phase velocity. Generally in such arrangements a substantial portion of energy fed by a transducer to the coupling device, which is a prism, undergoes successive reflections on the faces of the prism, this giving rise to delayed parasitic signals, which are injected into the acoustic line or into the transducer. Such parasitic signals are detrimental to the operation of the arrangement and it is an object of the invention to eliminate this drawback.

According to the invention there is provided a coupling device for exciting surface waves in an acoustic structure having a flat top, said device comprising: a body of dissipative material having two curved faces having in common a first and a second focus, an input and an output face, one of said two last mentioned faces being adjacent to one of said curved faces and the other intersecting said curved faces; said output face being coupled to said flat top; said input face being coupled to a transducer for radiating energy towards said output face with an angle of incidence on said output face such that the energy is reflected on said output face in a direction intersecting one of said curved face and containing one of said foci.

For a better understanding of the invention and to show how the same may be carried into effect, reference will be made to the drawing accompanying the following description and in which:

FIG. 1 is an explanatory diagram;

FIG. 2 is a view in perspective of a device according to the invention; and

FIG. 3 is an explanatory diagram relating to a modification.

For exciting a surface wave in an acoustic structure, an electromagnetic transducer which transmits a substantially plane and uniform wave is used. If the transducer. is fixed directly to the acoustic structure, an insufficient excitation is obtained, because the energy is propagated substantially in depth and not along the surface. In fact, only the periphery of the transducer participates actively in the excitation of the surface wave.

In order to overcome this drawback, the transducer is connected with the acoustic structure through a coupling 3,534,300. Patented Oct. 13., 1970 wedge; the wave transmitted by this wedge is propagated under oblique incidence so as to excite the structure by means of one of its components, having the same direction and the same phase velocity as the surface wave.

FIG. 1 shows in section an electro-acoustic transducer I placed on the input face BC of a coupling device 2. This device is cut in an elastic and energy dissipating ma terial such as a plastic material wherein the phase velocity is V It is mounted by its output surface DE on an acoustic structure 3 in which it excites a surface wave whose phase velocity is V It is known that to insure that a miximum of energy should be passed to structure 3, the angle of incidence 6 of the wave emitted by the transducer 1 with respect to the normal to surface ED has to be such that;

According to the invention, the output face ED of the coupling device 2 is a plane inscribed in a parabolic cylinder. The trace of this cylinder in the plane of the drawing is CDEO. It is a parabola whose axis is 0-2: and whose focus is F, i.e. the trace of the focal line of the parabolic cylinder. The axis 0-x, perpendicular to 0-y is therefore tangent at O to this parabola and parallel to the input face BC of the coupling device 2. The opposite surface of the coupler is also cut in the form of a parabolic cylinder, having as trace AB a second parabola with the axis 0-y and the focus F. The end of the coupler may extend beyond the axis of symmetry of the parabolas.

The wave transmitted by the transducer 1 parallel to axis 0-y impinges with an angle of incidence 0 on the output surface of the device 2, which contacts the acoustic structure 3. The surface wave is excited in this struc ture by the component of the incident wave which is propagated tangentially to the face DE. However, a fraction of the incident vibratory energy undergoes a first reflection on the acoustic structure 3 and further subsequent internal reflections on the parabolic surfaces AB and 0C. These internal reflections build up within device 2 a saw-tooth propagation path whose linear portions are all the less spaced from each other as they approach asymptotically the face A0 of the prism. This propagation path is so arranged that the fraction of the vibratory energy reflected on the acoustic structure 3 cannot reach the input and output faces of the coupler and is transformed into heat due to losses occurring within the coupler substance. The propagation path shown on FIG. 1 can be easily drawn from the consideration of the reflecting properties of parabolae AB and 0C, which are conic sections having the same two foci, i.e. a principal focus F and a secondary focus lying at infinity in the direction 0-y. It is well known that a wave emerging from one of the foci of a conic section is reflected toward its other focus and vice versa. On FIG. 1, the wave from the transducer generates on face DE a reflected wave travelling toward focus F; this reflected wave impinges on parabola AB and is reflected therefrom in a direction parallel to 0-y, since the secondary focus of parabola AB is at infinity in that direction. The Wave reflected on parabola AB travels toward parabola OC which reflects it toward focus F, and so on. It is readily seen that an endless folded path is obtained'along which the reflected energy can be safely repelled from face ED.

Obviously the excitation device according to the invention makes it possible to excite any acoustic structure on the surface of which a surface wave is to be propagated.

FIG. 2 shows, by way of non-limitative example, a view in perspective of an acoustic dispersive line formed by a thin sheet 3 integral with a base 4, and constructed to propagate on the surface a dispersive wave such as those known as Loves wave. This wave, which corresponds to a transverse vibration of the sheet 3, is excited by an electro-acoustic transducer whose deformations produce shearing forces. The transducer 1 excites the structure 34 through a coupling device 2, whose output face DE is tangent to the said structure. The reflection which takes place on this face is damped between the parabolic faces AB and OE. FIG. 2 shows only one end of the dispersive line, and the other end is equipped with an identical coupler and a receiving transducer.

The damping obtained within the coupler device according to the invention remains effective over a wide frequency range, which makes it possible to use it within a wide frequency range. This results from the very principle of progressive damping which leads to the reflected wave being completely absorbed before it reaches the axis of the parabolic cylinders which bound the coupling device. FIG. 3 shows a coupling device 2 substantially as shown in FIG. 2; its faces BC and ED are respectively coupled to the top of an acoustic structure 3 and to the radiating face of an electromechanical transducer 1. As disclosed above, the coupling device 2 has two curved faces AB and OC shaped as parabolic cylinders having the same two focal lines represented on FIG. 3 by a point P and another point at infinity in the direction -y. The vibratory energy emerging from transducer 1 impinges on the top of structure 3 so that a portion thereof is reflected in a direction parallel to 0y. This reflected portion of the vibratory energy undergoes another reflection on parabola DC from which it emerges in the direction of focus F; the wave reflected on parabola OC undergoes a further reflection on parabola AB and so on.

The modification shown in FIG. 3 is entirely similar to the embodiment shown in FIGS. 1 and 2 either in structure or operation except that the transducer 1 is coupled to the curved face OC, while the flat face BC is coupled to the top of the acoustic structure 3.

Of course the faces of the coupling device need not be parabolic. They may assume any other curved shape, provided the curve concerned has two foci. For example the curves may be elliptic.

What is claimed is:

1. A coupling device for exciting surface waves in an acoustic structure having a flat top, said device comprising: a body of dissipative material having two curved faces having in common a first and a second focus, an input and an output face, one of said two last mentioned faces being adjacent to one of said curved faces and the other intersecting said curved faces; said output face being coupled to said fiat top; said input face being coupled to a transducer for radiating energy towards said output face with an angle of incidence on said output face such that the energy is reflected on said output face in a direction intersecting one of said curved faces and containing one of said foci.

2. A coupling device as claimed in claim 1, wherein said foci are focal lines.

3. A coupling device as claimed in claim 2, wherein said output face is inserted in one of said curved faces.

4. A coupling device as claimed in claim 3, wherein said input and output faces form an angle and the sine of said angle is equal to the ratio of the respective phase velocities of the acoustic waves in said body and in said structure.

5. A coupling device as claimed in claim 1, wherein said curved faces are parabolic cylinders.

6. A coupling device as claimed in claim 1, further comprising the electro-acoustic transducer coupled to said input face.

7. A coupling device as claimed in claim 1, wherein said input face is said other face intersecting said curved faces.

8. A coupling device as claimed in claim 4, wherein said transducer is a shear-wave transducer.

9. A coupling device as claimed in claim 1 wherein said input face is said one face adjacent to one of said curved faces.

References Cited UNITED STATES PATENTS 2,799,191 1/1937 Willard 73-67.8 3,353,120 9/1967 Tournois 333-30 3,317,862 5/1967 Fitch 333-30 3,289,114 11/1966 Rowen 333-30 3,103,640 9/1963 Lockhart 333-30 3,070,761 12/1962 Rankin 333-30 2,649,550 12/1953 Hardie 310-82 HERMAN K. SAALBACH, Primary Examiner C. BARAFF, Assistant Examiner US. Cl. X.R. 

